The present invention relates to the field of focused ion beam systems and, in particular, to sample observation and registration in such systems.
Focused Ion Beam (FIB) systems are widely used in microscopic-scale manufacturing operations because of their ability to image, etch, mill, deposit, and analyze with great precision. Ion columns on FIB systems using gallium Liquid Metal Ion Source (LMIS), for example, can provide five to seven manometer (10xe2x88x929m) lateral resolution. Because of their versatility and precision, FIB systems have gained universal acceptance in the integrate circuit industry as necessary analytical tools for use in process development, failure analysis, and most recently, defect characterization.
During the manufacturing of integrated circuits, multiple copies of an integrated circuit are fabricated on a semiconductor silicon wafer, which is then severed into rectangular dies, each die including a copy of the integrated circuit. A die is typically several hundred microns thick and has electronic circuit elements fabricated on and slightly below its front side surface. The ion beam of a FIB system scans the surface of the integrated circuit in a raster pattern. This raster pattern produces an image of the surface showing the top lines and elements of the circuit. The image is used to navigate the ion beam around the die to locate a specific element or a feature of the circuit. Upon moving the raster pattern to the local area of the feature of interest and increasing the ion beam current, the ion beam will cut into the die and expose circuit features in buried layers. The FIB system can then alter the exposed circuit by cutting conductive traces to break electrical connections or depositing conductive material to provide new electrical connections. FIB systems often include a Secondary Ion Mass Spectrometer (SIMS), which can determine which chemical elements are present in the exposed features. Technology in the semiconductor industry evolves rapidly, however, and existing tools and techniques may be inadequate for use with new integrated circuit designs.
In use, a semiconductor die is mounted in a package. The package has metal leads for electrically connecting it to a circuit board on which other electrical components are mounted. The die and the package both have bonding pads for establishing electrical connections between the die and the package leads. The connection between the package leads and the die inherently adds undesirable impedance, that is, resistance to electrical flow, between the package leads and the die. As semiconductor devices operate at higher speeds, lower impedances between the package leads and the active elements on the die are required. At the same time, larger and more complex devices and circuits require an increased number of input/output connections, resulting in larger die size and packages.
A response to these needs has been the development of so-called flip-chip or C4 semiconductor manufacturing technology in which the bond connections are arrayed over the front side of a die and align with an array of corresponding bond connections on the package. The die is then placed front side down in the package, with the bond connections on the die contacting those on the package. The connection length and impedance from the active circuit elements to the package pins are reduced, compared with those of older connection technologies, and the number of connections available between the die and the package is increased. 
The top layer of the flip-chip die, however, is covered with an array of bond pads, making access to circuit elements from the front surface of the die difficult or impossible, even for unpackaged devices. Thus, conventional analysis and repair tools are often unusable with flip chips. Improved techniques to debug flip-chip devices without damaging them are needed to remove a crucial roadblock in the advancement of flip chip technology and to provide a significant boost in time-to-market for critical IC chips, such as microprocessors. With improved techniques, new circuits can be made and tested within days, not months of receiving a prototype.
Some techniques exist for debugging flip-chips. U.S. Pat. No. 5,821,549 to Talbot et al., for example, describe using laser milling to thin the flip-chip from the backside by cutting a series of steps in the substrate. An infrared optical microscope is then used to locate features of interest on the thinned portion of the flip-chip. The edges of the laser drilled steps are used to register the infrared image with a FIB image, and the FIB is then used to accurately mill down to the feature desired. Thus, the operation requires the use of three instruments, a laser, an optical microscope, and a FIB. Registering the infrared image with the FIB image is difficult, time consuming, and the images are subject to misalignment. Thus, to do efficient FIB operations on flip-chip features, there exists a need for a fast and accurate method of locating features through the back surface of the flip-chip.
In integrated circuit operations, as well as in other applications, a FIB operator typically locates a feature of interest by scanning the ion beam over the specimen while using the imaging to view the specimen. A disadvantage of FIB systems is that the focused ion beam incidentally etches and implants gallium ions in the substrate as it is imaging. Thus, there is a need for a fast and accurate method for aligning an ion beam with specific features on any specimen, particularly ones having sensitive surfaces that can be damaged by the focused ion beam, while minimizing undesirable exposure of the specimen to the ion beam.
Thus, it is an object of the invention to provide an improved method and apparatus for locating specimen features in a focused ion beam system.
It is another object of the invention to provide such a method and apparatus that minimizes exposure of the specimen to the ion beam.
It is still another object of the invention to provide a focused ion beam system, having an optical microscope for locating specimen features and provide coaxial alignment between the two.
It is a further object of the invention to permit an operator to align a specimen visually and then perform an ion beam operation on the specimen, without moving the specimen to a second instrument or requiring registration with pre-recorded images.
It is yet another further object of the invention to provide a focused ion beam system having a coaxial optical microscope for use with infra-red, visible, or other frequencies of light.
It is still another object of the invention to provide an improved method and apparatus for using a focused ion beam system to operate on flip-chips.
It is a still further object of the invention to provide a method of machining thin-film heads in a focus ion beam system while minimizing the damage to the heads from ion beam imaging.
It is yet a further object of the invention to provide a means for optically processing an integrated circuit with a focused ion beam system.
The present invention comprises a focused ion beam system that includes an optical microscope having an optical axis that substantially coincides with the axis of the focused ion beam as the axes approach a specimen. The axis of the focused ion beam is defined as a line at the center of the beam when positioned at the point defined as zero deflection.
Features on the specimen are located using the image from the optical microscope. When a feature is centered in the image of the optical microscope, the feature will be in the path of the ion beam, when activated. Because the optical image and the FIB image are aligned, the operator can use the optical image to position the ion beam without having to go through a lengthy, less accurate registration procedure that relies on a recorded image and accurate repositioning of a movable specimen stage. With the present invention, the operator uses a live image from the optical microscope to align the focused ion beam with features on the specimen.
The optical microscope can use light of different frequencies for different applications. For example, an optical microscope can use infrared light for viewing features through a layer of silicon or visible light frequencies for viewing a surface feature or a feature under a layer of transparent silicon dioxide. The optical microscope can also direct light from a laser or other light source onto a specimen to process the specimen. The scope of optical processing of the sample in conjunction with the ion beam and gas chemistry is large. Optical Beam Induced Current, OBIC, is one example of measuring the thickness of material as the ion beam is removing the material. Another example is using a laser beam through the optical microscope to heat the sample in a local area under the ion beam. This heating will enhance the gas chemistry between the ion beam and process gasses. In a preferred embodiment, the optical microscope uses an angled mirror at the end of the ion column to reflect light from a specimen into image-forming optical elements. The mirror includes an aperture hole through which the ion beam travels. The preferred embodiment also includes one or more illumination sources and a camera for recording the image and presenting it on a monitor to an operator. One embodiment allows the operator a choice between dark field illumination, provided by light sources in the vacuum chamber or bright field illumination, provided along the optical axis.
Thus, the invention speeds and simplifies locating features on the specimen. Using the optical microscope to locate a feature and position the specimen reduces exposure of the specimen to the scanning focused ion beam, thereby reducing undesirable etching of the specimen, which can damage sensitive specimens, such as thin films. The optical microscope allows rapid positioning of the parts in relation to the ion beam, thus increasing the number of parts that can be machined in a time period and improving the quality of those parts.
The invention is particularly suitable for performing focused ion beam operations on flip chips. Using infrared light, it is possible to look through 100 xcexcm or more of silicon, thereby allowing an operator to locate, from the backside of a thinned die, features that can then be exposed by ion beam milling for other FIB operations.
Additional objects, advantages and novel features of the invention will become apparent from the detailed description and drawings of the invention.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.